1. Field of the Invention
The present invention relates to a rotary kiln for roasting cement raw meals and a method for driving the rotary kiln. More particularly, it relates to a rotary kiln sloped more gently and rotating at a higher speed than conventional rotary kilns provided with, on the raw meal inlet side of the kiln, a suspension preheater having a calcining furnace, thereby improving the granulation of the cement raw meals therein.
2. Description of the Prior Art
FIG. 1 shows an apparatus, for roasting cement raw meals, which comprises a suspension preheater 1 having a vertical calcining furnace 2 connected to staged cyclones C.sub.1 -C.sub.4, a rotary kiln 3 for forming clinker and a cooler 4 for cooling the clinker. In the roasting apparatus, the raw meals A are gradually preheated by a hot gas, proceeding up through the gas duct 7, induced by a waste gas fan 8, as the meals proceed downwards through a series of cyclones C.sub.1 -C.sub.3. (The raw meal flow is indicated by a dotted arrow line and the hot gas flow by a solid arrow line.) The raw meals are then fed into the calcining furnace 2 in which hot gas is introduced from the cooler 4 through a duct 13 and fuel is supplied thereto from a burner 6a to thereby calcine the raw meals. The calcined raw meals enter into the last stage cyclone C.sub.4 followed by introduction into the rotary kiln 3 through a transition housing 12 connected to the calcining furnace 2.
To the rotary kiln 3, hot gas from the cooler 4 and fuel from the burner 6b are supplied. While the raw meals are transported from the feed inlet of the rotary kiln 3 to the outlet, they are roasted into clinker at an elevated temperature in the rotary kiln 3. The resulting clinker is discharged from the kiln 3 to the cooler 4 in which while the clinker is carried on a lattice 14 it is cooled by cold air supplied thereto by a blower 10 and crushed by a crusher 15 and then carried out by means of a conveyor 16 or the like. The surplus air in the cooler 4 is drawn through a dust collector 17 by means of an induced draft fan 9 to separate the fine clinker dust from the surplus air which is combined with the product clinker.
The rotary kiln 3, being lined with refractory materials, is supported with a slight inclination to the horizontal by a supporting means (not shown) and rotates around its axis by a driving means (not shown).
The grain size of the clinker formed in the rotary kiln 3 varies widely ranging from massive chunks of clinker to fine powder-like clinker. These sizes of clinker are unevenly distributed and thermal hysterisis is unevenly applied during the cascading motion in the kiln. The massive chunks of clinker tend to be excessively roasted on its outer surface, but tend to be insufficiently roasted in the center of the chunks. The fine powder-like clinker tends to be insufficiently roasted as well. Thus, these clinkers are unevenly roasted as a whole.
Further, when the clinkers are supplied from the rotary kiln 3 to the cooler 4, as shown in FIG. 2, they tend to be segregated according to their grain size when discharged to the cooler 4 due to the cascading motion within the rotary kiln 3 rotating in the direction of the arrow mark thereby accumulating in layers on the lattice 14 with the fine clinker 19a distributed at the side wall 4a of the cooler 4. The segregated fine clinker is transported in the longitudinal direction to form a strip-like fine clinker layer 19a on the lattice 14 along the side wall 4a.
In the area from the center to the other side wall 4b, a grained clinker layer 19b comprised of larger and coarser grains of clinker is formed, which is easily cooled due to the fact that a large amount of cooling air can pass through the layer consisting of such coarse grains. The fine clinker layer 19a however is insufficiently cooled due to the fact that the individual particle size is so fine that enough cooling air can not pass through such a layer. In addition, the fine clinker tends to be fluidized to flow in the direction to the outlet of the cooler 4 by the cooling air passing therethrough, resulting in the formation of a fine and hot clinker flow which is known as "Red River". This flow reduces a cooling efficiency of the cooler 4, raises the temperature of the clinker discharged from the cooler 4, increases fuel consumption in the roasting process, and damages the lattice 14 and the side wall of the cooler 4 by overheating them. Reference number 20 indicates a conveyor for transporting the finest of this powder-like clinker which falls from the apertures of the lattice 14. On the contrary, even when the massive chunks of clinker arrive at the oultet of the cooler, the interior is still red-hot so that when these large chunks are crushed by the crusher 15 the hot clinker becomes mixed with the cooled clinker, resulting in raising the average temperature of the product clinker.
As mentioned above, both the massive chunks of clinker and the fine powder-like clinker have an undesirable effect on the cooling efficiency and a thermal recovery of the cooler. For example, the nonuniform roasting and the insufficient cooling in the roasting process lead to a decrease in the grinding capacity and an increase in the grinding energy consumption in the succeeding cement grinding process. Thus, it is important to form clinker having a grain size of as narrow a distribution as possible in the rotary kiln, reducing the amount of the massive chunks and the fine powder-like clinker as much as possible.
However, the conventional rotary kiln provided with, to the raw meal inlet side of the kiln, a suspension preheater with a calcining furnace is designed in view of heat transmission alone without taking account of the function of granulating the raw meals therein. The slope and the rotational speed of the rotary kiln are selected so as to maintain a proper residence time of the clinker in the rotary kiln to receive a required heat from hot gas. Generally, the slope of such a conventional rotary kiln is in the range of approximately 3.5% to 4% and the rotation speed is in the range of 2.5 rpm; to 3.5 rpm; at a maximum. It is believed that power consumption and the life of the rotation parts of the kiln are shortened if the slope become more gentle and at the same time the rotational speed becomes greater than the aforementioned.